Dynamics of Innate Immune Response Due to Bacteria-Induced Pulpitis

IntroductionPulpitis is associated with dental carries and can lead to irreversible pulp damage. As bacteria penetrate deeper into dentin and pulp tissue, a pulpal innate immune response is initiated. However, an understanding of the types of immune cells in the pulp, how this relates to bacterial infiltration, and the dynamics of the immune response during pulpitis is limited. As conserving the vitality of the pulp tissue through conservative therapies becomes an important part of dental practice, there is a greater need to understand the kinetics and composition of the immune response during pulpitis. MethodsDental pulp exposure in molars of mice was used as an animal model of pulpitis. To investigate the kinetics of immune response, pulp tissue was collected from permanent molars at different time points after injury (baseline, day 1, and day 7). Flow cytometry analysis of CD45+ leukoctyes including macrophages, T cells, neutrophils and monocytes was performed. 16S in situ hybridization captured bacterial invasion of the pulp, and immunohistochemistry for F4/80 investigated spatial and morphological changes of macrophages during pulpitis. Data were analyzed using two-way ANOVA with Tukeys multiple comparisons. ResultsBacteria mostly remained close to the injury site, with some expansion towards non-injured pulp horns. We found that F4/80+ macrophages were the main immune cell population in healthy pulp. Upon injury, CD11b+Ly6Ghigh neutrophils and CD11b+Ly6GintLy6Cint monocytes constituted 70-90% of all immune populations up to 7 days after injury. Even though there was a slight increase in T cells at day 7, myeloid cells remained the main drivers of the immune response. ConclusionsAs bacteria proliferate within the pulp chamber, innate immune cells including macrophages, neutrophils and monocytes predominate as the major immune populations, with minimal signs of transitioning to an adaptive immune response.